Optical device with variable aperture

ABSTRACT

An optical device with variable aperture, including a deformable membrane having a central optical area, a support to which a peripheral anchoring area of said membrane is connected, a first cavity filled with a constant volume of a first transparent fluid in a determined range of wavelengths, said cavity being delimited at least in part by a first face of said membrane and a wall of the support. The optical device also includes at least one actuation device of a region of the membrane located between the peripheral anchoring area and the central optical area of the membrane, configured to bend said region of the membrane by application of electrical actuation voltage so as to displace some of the volume of the first fluid towards the center or towards the periphery of the first cavity, said displacement of fluid being intended to deform the central area of the membrane.

FIELD OF THE INVENTION

The present invention relates to an optical device with variableaperture, also called diaphragm, and a manufacturing method of such adevice.

BACKGROUND OF THE INVENTION

A diaphragm is a mechanical element interposed on the path of a lightbeam in an optical system for defining the amount of light transmittedand the aperture of the system.

Such a device is especially used in high-performance imaging systems asit can ensure control functions of the light flow or regulation of thedepth of field. It can also block diffracted rays in the optical systemand minimize aberrations of the optical system.

The iris diaphragm, described in document U.S. Pat. No. 21,470 [1], isstill widely used in recent and evolved optical systems. It comprises anassembly of mobile blades of variable number depending on lens size. Amechanism turns the blades and therefore regulates the aperture.

Several limitations are associated with this type of diaphragm.

First of all, this is a complex and expensive solution. The complexityof the mechanical structure (mechanism for displacement of blades)creates assembly difficulties. Also, a sufficient number of mobileblades has to be integrated to obtain a quasi-circular aperture(generally necessary for an optical system). The manufacturing cost ofsuch a diaphragm is high and this technological solution thereforeproves expensive.

Also, the power consumed by such a device is high. In effect, the forcerequired for changing the aperture is impacted by the friction betweenthe mobile mechanical pieces. It is therefore necessary to use powerfulmotors to modify the aperture.

In addition, the complex mechanical structure and the motors usedprovide a particularly bulky device.

Finally, wear of the mobile mechanical pieces limits the reliability ofthe diaphragm over time.

Taking the same approach as the iris diaphragm, novel mechanicalsolutions have been developed in recent years [2, 3].

Of these solutions mechanical, some are based on actuation by MEMS(micro-electromechanical systems) to optimize size and reduceconsumption. Such is the case in references [2, 3] mentioned above.

However, these solutions fail to overcome all limitations of the irisdiaphragm. In effect, these mechanical diaphragm technologies all havelarge dimensions and manufacturing complexity mainly connected with thesame operating principle. Also, the majority of them do not produce aquasi-circular aperture required for optical systems.

For several years now, novel non-mechanical solutions have beendeveloped.

In particular, several fluidic solutions have been developed as analternative to mechanical solutions. State of the art of diaphragms withvariable aperture with fluidic structure is detailed in the thesis byPhilipp Müller [4], a brief synthesis of which is presented herein belowin reference to FIGS. 12A to 12G. Each of these figures represents thesame respective device in two aperture configurations.

In the technological solution presented in FIG. 12A, the optical device200 comprises a plurality of semi-spheres 201 made of transparentelastomer pressed against a substrate 202 made of PMMA and encapsulatingan opaque liquid 203. The pressure and amount of opaque liquid locatedbetween the semi-spheres 201 and the substrate 202 are adjusted so as tomore or less let light through and adjust the aperture diameter. Forthis purpose, the device comprises an inlet 204 of opaque liquid coupledto a system such as a pump (not shown) external to the device 200. Thisdevice is particularly significant in creating an array of diaphragms.However, such a device is not integrated, as the system for pressurizingthe opaque liquid is placed to the exterior of the device, such thatthis solution is bulky.

The device 300 illustrated in FIG. 12B comprises a deformable membrane301 and a constant volume of opaque liquid 302 contained in a firstcavity defined in part by the membrane 301. On the face opposite theopaque liquid 302, the membrane 301 is in contact with gas 303, forexample air, contained in a second cavity. Said second cavity comprisesa gas inlet 304 coupled to an external system (not illustrated) forpressurizing said gas. More or less substantial pressure is applied tothe membrane 301 by the gas 303 introduced to or withdrawn from thecavity via the inlet 304. The opaque liquid 302 is pushed by themembrane 301, varying the aperture of the diaphragm. This produces thesame disadvantage as for the previous solution (non-integrated solution)due to the pressurizing system being external.

The device 400 illustrated in FIG. 12C comprises an opaque liquid 401trapped between a glass substrate 402 and a deformable membrane 403actuated by an annular piezoelectric actuator arranged at the peripheryof the membrane. Under the effect of the actuator, the opaque liquid 401is pushed from the center of the device and the center of the membraneis pressed progressively on the substrate 402. This solution isintegrated, the opaque liquid 401 being encapsulated at constant volume,without it being necessary to provide an inlet/outlet for liquid or anouter pressurizing system). However, the dimension is still considerable(of the order of 25 mm per side). A major disadvantage to this solutionis the resulting mediocre optical quality. In effect, when the membrane403 is pressed against the substrate 402, a small amount of opaqueliquid can remain locally which compromises the optical quality of theensemble. Also, the solid/solid interface between the membrane and theglass generally produces a large error on the wavefront and mediocreoptical quality of the bandwidth of the diaphragm. Once the membrane 403and the substrate 402 are in contact, adhesion between the tworespective materials can complicate or even prevent reverse operationand return of the opaque liquid over all or part of this area.

The device 500 illustrated in FIG. 12D comprises a plurality ofconcentric micro-channels 501 and an intake 502 for opaque liquid 503.Similar to the devices of FIGS. 12A and 12B, this solution is notintegrated.

The device 600 illustrated in FIG. 12E comprises an opaque liquid 601and a liquid 602 transparent to the light beam to be transmitted, aswell as two inlets (601 a, 602 a)/outlets (601 b, 602 b) for each ofsaid liquids. This system is highly complex and achieves small aperturevariations only. Also, the volume of liquid in the device is notconstant and systems external to the device are necessary for operationto ensure the laminar flow of both liquids.

In the examples illustrated in FIGS. 12F and 12G, the device 700,respectively 800 comprises two liquids, one opaque and the othertransparent to the light beam to be transmitted, and electrodes foradjusting the wettability of one of said liquids. The operatingprinciple in these two cases is based on electro-wetting, a techniquewell known in the field of fluidics. In the case of FIG. 12F, atransparent electrode 701 made of ITO is sufficient to vary thewettability of the opaque liquid 703 (and therefore its radius ofcurvature) relative to a hydrophobic dielectric material 702 and to moreor less open the central area of the device (the transparent liquidbeing designated by the marker 704). In the case of FIG. 12G, theprinciple is the same but the sole electrode is replaced by severalinterdigitated electrodes 801 to best control the form of the interfacebetween the respectively transparent 803 and opaque 804 liquids (thehydrophobic dielectric being designated by the marker 802). In boththese cases, the solution is integrated (liquids are encapsulated atconstant volume, no need of inlet/outlet nor complementary outersystem). The disadvantages relative to both these solutions are thesignificant thickness of the device (typically 2 mm) and the strongelectrical power supply required (typically 100 V). This lattercharacteristic the makes the use and control of the device complex andimpacts significantly the cost of the solution.

BRIEF DESCRIPTION OF THE INVENTION

An aim of the invention is therefore to design an optical device withvariable aperture which is more compact than existing devices, which isless expensive to manufacture and which involves low power consumption.

According to the invention, an optical device with variable aperture isproposed, comprising:

-   -   a deformable membrane comprising a central optical area,    -   a support to which a peripheral anchoring area of said membrane        is connected,    -   a first cavity filled with a constant volume of a first        transparent fluid in a determined range of wavelengths, said        cavity being delimited at least in part by a first face of said        membrane and a wall of the support,    -   at least one actuation device of a region of the membrane        located between the peripheral anchoring area and the central        optical area of the membrane, configured to bend said region of        the membrane by application of electrical actuation voltage to        displace some of the volume of the first fluid towards the        center or towards the periphery of the first cavity, said        displacement of fluid being for deforming the central area of        the membrane,

said optical device being characterized in that it also comprises aconstant volume of opaque liquid in said determined range ofwavelengths, in contact at least locally with a second face of themembrane opposite the first face and with a second transparent fluid insaid determined range of wavelengths and non-miscible with said opaqueliquid.

Particularly advantageously, the volume of opaque liquid is selected sothat:

-   -   in a rest situation when no electrical voltage is applied to the        actuation device, the opaque liquid covers at least part of the        membrane to produce an aperture having a first diameter, and    -   in an actuation situation when a non-zero electrical voltage is        applied to the actuation device, with the central part of the        membrane having a curvature different to the curvature at rest,        the opaque liquid covers at least one part of the membrane to        produce an aperture having a second diameter different from the        first diameter.

The aperture of this optical device can optionally be zero, in whichcase the optical device completely blocks the optical field and can besimilar to a shutter.

According to other advantageous characteristics of the invention,considered singly or in combination:

-   -   the device further comprises a second cavity opposite the first        cavity relative to the membrane, said second cavity containing        the opaque liquid and a constant volume of the second        transparent fluid;    -   the opaque liquid and the second transparent fluid have        substantially the same density;    -   the first and second transparent fluids have substantially the        same refraction index;    -   the first and the second cavity have a transparent wall opposite        the membrane;    -   the transparent wall of the first and/or of the second cavity        comprises an optical filter on its face opposite the cavity;    -   the transparent wall of the first and/or of the second cavity        comprises fixed optics on its face opposite the respective        cavity;    -   the transparent wall of the first and/or of the second cavity        comprises a device with variable focal length;    -   said wall can have a central aperture and said device with        variable focal length comprises:        -   a deformable membrane closing said aperture, a peripheral            area of the deformable membrane being anchored on said wall,        -   at least one actuation device of a region of the membrane            located between the peripheral anchoring area and the            central area of the membrane, configured to bend said region            of the membrane by application of electrical actuation            voltage so as to displace some of the volume of the fluid            towards the center or towards the periphery of the cavity;    -   the membrane comprises a stiffening structure comprising cells        which delimit, in the central optical area of said membrane, at        least two deformable regions;    -   the second transparent fluid is arranged in the second cavity in        the form of a plurality of elementary volumes each arranged        facing a respective cell;    -   the second transparent fluid is arranged in the form of a single        continuous volume facing the cells;    -   the second transparent fluid is arranged on the wall of the        second cavity opposite the deformable membrane in the form of a        plurality of elementary volumes;    -   the actuation device is piezoelectric.

Another object relates to a manufacturing method of such an opticaldevice with variable aperture.

Said method comprises the following steps:

-   -   providing a device with variable focal length comprising the        deformable membrane, the actuation device and the first        transparent liquid in the first cavity,    -   dispensing a determined volume of the opaque liquid on the        deformable membrane.

Particularly advantageously, in the event where the optical device isprovided with a second cavity containing the opaque liquid and thesecond fluid transparent, the method comprises the following steps:

-   -   providing the optical device with variable focal length obtained        by the method such as described hereinabove,    -   providing a second substrate and dispensing the second        transparent fluid on said second substrate in the form of at        least one drop,    -   adhesion of the second substrate on the optical device with        variable focal length, so as to encapsulate the second        transparent fluid and the opaque liquid between the second        substrate and the deformable membrane.

BRIEF DESCRIPTION OF DRAWINGS

Other characteristics and advantages of the invention will emerge fromthe following detailed description in reference to the appendeddrawings, in which:

FIGS. 1A and 1B are sectional views of an optical device according to anembodiment of the invention, respectively at rest and in the actuatedstate,

FIG. 2 is a view similar to FIG. 1B, illustrating displacements of thefirst transparent fluid and of the opaque liquid during actuation of thedevice,

FIGS. 3A to 3C are sectional views of an optical device according to anembodiment of the invention in which the aperture has a non-zerodiameter at rest, respectively at rest and in two possible actuationconfigurations,

FIGS. 4A to 4C are sectional views of an optical device according todifferent embodiments integrating various additional functionalities,

FIGS. 5A and 5B are sectional views of an optical device according to anembodiment of the invention in which the opaque liquid and the secondtransparent fluid are not encapsulated in a cavity, respectively at restand in the actuated state,

FIGS. 6A to 6D illustrate different configurations of the optical deviceaccording to the invention at rest,

FIGS. 7A and 7B are sectional views of an optical device at restaccording to two embodiments of the invention in which the wettabilityof the second transparent fluid relative to the wall of the secondcavity is different,

FIGS. 8A and 8B are sectional views of an optical device comprising anetwork of diaphragms according to an embodiment of the invention inwhich the membrane comprises a stiffening structure and the secondtransparent fluid is arranged in the form of a network of droplets,respectively at rest and in the actuated state,

FIGS. 9A and 9B are sectional views of an optical device comprising anetwork of diaphragms according to an embodiment of the invention inwhich the membrane comprises a stiffening structure and the secondtransparent fluid is arranged in the form of a single continuous volume,respectively at rest and in the actuated state,

FIGS. 10A and 10B are sectional views of an optical device comprising anetwork of diaphragms according to an embodiment of the invention inwhich the second transparent fluid is arranged in the form of a networkof droplets, respectively at rest and in the actuated state,

FIGS. 11A to 11E illustrate different steps of the manufacturing of anoptical device with variable aperture according to an embodiment of theinvention,

FIGS. 12A to 12G illustrate optical devices with variable aperturebelonging to the prior art.

For reasons of clarity of the figures, the different elementsillustrated are not necessarily shown to the same scale.

From one figure to the other, identical reference numerals designatesimilar elements which are therefore not described in detail for eachnew figure.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1A and 1B illustrate an embodiment of an optical device 100 withvariable aperture according to the invention, respectively at rest (thatis, in the absence of application of electrical voltage), in this caseproducing a zero aperture, and in the actuated state (electricalactuation voltage being applied), in this case producing an aperturehaving a non-zero optical diameter D.

Said device 100 comprises a deformable membrane 1 comprising a centralarea 1 a which defines an optical field of the device and a support 10,12 to which a peripheral anchoring area 1 c of said membrane isconnected.

The membrane and a wall of the support delimit at least in part a firstcavity which is filled with a constant volume of a first transparentfluid 2 in a determined range of wavelengths. Said range of wavelengthstypically comprises the range of wavelengths which must be transmittedthrough said optical device. The membrane 1 is in contact, via a firstmain face, with the first fluid 2. The membrane is also transparent insaid range of wavelength.

Membrane means any supple and tight film such that the membrane forms abarrier to two fluids located on either side of the membrane.

The first transparent fluid 2 is sufficiently incompressible to be movedtowards the central part of the first cavity when force is applied tothe membrane in the direction of said fluid (and inversely, towards theperiphery of the first cavity when force is applied to the membrane in adirection opposite said fluid), this force being applied in anintermediate part between the anchoring area and the central part of themembrane.

The shape of the support and of the membrane can advantageously have ashape of revolution about the optical axis of the optical device, butthose skilled in the art could select any other shape without as suchdeparting from the scope of the present invention.

On the other hand, the second main face of the membrane 1—opposite thefirst face—is in contact with a constant volume of opaque liquid 3 insaid determined range of wavelengths. Said opaque liquid is further incontact with a second transparent fluid 4 in said determined range ofwavelengths and non-miscible with said opaque liquid.

In the embodiment illustrated in FIGS. 1A and 1B, the opaque liquid 3and the second transparent fluid 4 are contained together in a secondcavity located on the other side of the first cavity relative to themembrane 1. The second transparent fluid 4 has a constant volume.

However, as will be evident in another embodiment described later (cf.FIGS. 5A-5B), the opaque liquid and the second transparent fluid are notnecessarily contained in a specific cavity. It is in fact possible thatthe opaque liquid is kept in contact with the second main face of themembrane due to its wettability relative to the material of themembrane, and that the second transparent fluid is ambient air.

The optical device 100 also comprises at least one actuation device 5 ofa region 1 b of the membrane—so-called actuation region—located betweenthe peripheral anchoring area 1 c and the central optical area 1 a ofthe membrane.

Said actuation device 5 is configured to bend said region 1 b of themembrane by application of electrical actuation voltage so as todisplace some of the volume of the first transparent fluid towards thecenter or towards the periphery of the first cavity, said displacementof fluid being intended to deform the central area of the membrane bymodifying the fluid pressure exerted on said central area.

Those skilled in the art know different actuation devices may beutilized for actuating membranes.

These devices are based on different technologies, examples of which arepiezoelectric actuation, electrostatic, electromagnetic, thermalactuation or even based on electro-active polymers.

In this respect reference could be made to a detailed description ofsuch actuation devices used in optical devices with variable focallength described in documents [5]-[10].

The choice of actuation technology and sizing of the actuation devicedepends on expected performance levels (for example, electricalconsumption), stresses to which it will be subjected during operation ofthe device, and considerations relative to the electrical actuationvoltage to be applied.

For example, a particularly efficient actuation device is based onpiezoelectric technology.

It is recalled that a piezoelectric actuator comprises a block ofpiezoelectric material sandwiched totally or partially between twoelectrodes intended, when fed, to apply an electrical field to thepiezoelectric material. This electrical field is used to controlmechanical deformation of the block of piezoelectric material. The blockof piezoelectric material can be monolayer or multilayer and extendbeyond an electrode. PZT is preferably selected as piezoelectricmaterial.

The actuation device can comprise a single actuator in the form of acrown or else several separate actuators (for example in the form ofbeams) distributed uniformly over the circumference of the membrane.

Optionally, the actuators can be capable of bending in two oppositedirections.

The actuation device can be arranged on the inner face of the membrane,on the outer face or even inside the membrane.

Optionally, the actuation device can extend in part over the peripheralanchoring area.

At rest (FIG. 1A), the membrane 1 is planar and the opaque liquid 3forms a substantially uniform layer covering the second face of themembrane 1. In this situation, the opaque liquid prevents any incidentradiation on the device 100 to be transmitted. In other terms, theradius of the diaphragm formed in this way is zero.

When electrical voltage is applied on actuation device (FIG. 1B), thefirst transparent fluid 2 deforms the center of the membrane. In effect,said fluid is pushed to the center of the first cavity by the actuationdevice 5, exerts pressure on the central part 1 a of the membrane andchanges its radius of curvature. Conversely, the opaque liquid 3 ispushed towards the periphery of the membrane 1, in the actuation region1 b.

From a certain electrical voltage (whereof the value depends on thevolume of opaque liquid and geometry of the membrane), deflection of theactuation device 5 is sufficient to collect enough opaque liquid 3 torelease the center of the membrane. The central deformation of themembrane combined with the flow of opaque liquid towards the peripheryof the membrane progressively releases the center of the device byletting the membrane 1 make contact with the second transparent fluid 4.

The aperture of diameter D created in this way lets incident radiationthrough. The more accentuated the actuation (by increasing voltageapplied), the larger the aperture.

As best seen in FIG. 2, which represents the same device as FIG. 1B,arrows A represent displacement of the first transparent fluid 2 in thefirst cavity, and arrows B represent displacement of the resultingopaque liquid 3 in the second cavity.

As compared to existing solutions, the invention has many advantages:

-   -   the solution is fully integrated, that is, with no outer element        such as a pump or other and therefore has small size (from 3 to        10 mm per side, for example),    -   the proposed optical interface is of good quality. In fact, the        solid/fluid interface between the membrane and the second        transparent fluid is compatible with a satisfactory error on the        wavefront,    -   the thickness of such a device is optimized (typically 400 μm to        700 μm),    -   the required actuation voltage remains low (typically 15V for a        piezoelectric actuation device) and the associated power        consumption can be extremely low (of the order of 0.1 μW),    -   the response time corresponding to this invention is fast        (typically a few ms),    -   the manufacturing cost can be very competitive as it benefits        from collective manufacturing (waferlevel type).

In the example illustrated in FIG. 1A, the device at rest (0 V) has nooptical aperture (zero diameter). However, it is quite possible for thedevice at rest (0 V) to have an aperture of non-zero diameter D0, asillustrated in FIG. 3A.

In this case, this optical aperture can be increased to a diameter D1>D0(FIG. 3B) or decreased to a diameter D2<D0 (FIG. 3C) by applyingelectrical voltage to the actuation device, depending on the design ofactuation and its direction of deflection.

In the example illustrated in FIGS. 1A and 2A, the first cavity isdelimited by the membrane 1 and a first substrate 11 connected to themembrane by a peripheral support 10. Similarly, the second cavity isdelimited by the membrane 1 and a second substrate 13 connected to themembrane 1 by a peripheral support 12. The anchoring area of themembrane is included between the peripheral supports 10 and 12.

Said first and second substrates 11, 13 are transparent in the range ofwavelength in which the optical device 100 must transmit a light beam.Said substrates can be for example glass slides with parallel faces.

Advantageously, one at least of said substrates 11, 13 can also take onfunctions of optical filter, optical power on the device and/orvariation in focal length.

So, in the embodiment of FIG. 4A, the first substrate 11 is provided onits face opposite the first cavity with an optical filter 110,anti-reflecting and/or infrared.

In the embodiment of FIG. 4B, the first and the second substrate 11, 13are each provided with a respective fixed optic 111, 130 which alsocontributes constant optical power to the device 100.

In the embodiment of FIG. 4C, the first substrate 11 has a centralaperture 112 which is closed by a deformable membrane 1′. The membrane1′ is of the same type as the membrane 1 but can have differentdimensions and/or mechanical properties. The peripheral area 1 c′ of themembrane 1′ is anchored between the substrate 11 and the peripheralsupport 10. In addition, an actuation device 5′ of the membrane 1′ isarranged in an intermediate region 1 b′ between the central part 1 a′ ofthe membrane 1′ and the peripheral anchoring area. The device 5′ can beof the same type as the device 5 or be based on other actuationtechnology. The membrane 1′ and its actuation device 5′ vary the focallength of the device 100. In fact, as a function of the electricalactuation voltage applied to the device 5′, some of the firsttransparent fluid can be pushed towards the center or towards theperiphery of the first cavity, and modify the curvature of the centralpart 1 a′ of the membrane 1′.

Naturally, the functionalities present in the embodiments of FIGS. 4A-4Ccan be combined together or be incorporated into just one of thesubstrates without as such departing from the scope of the presentinvention.

The volume of opaque liquid and the wettability of said liquid relativeto the membrane are selected to enable the operation describedhereinabove, specifically a variation between:

-   -   a rest situation when no electrical voltage is applied to the        actuation device, the opaque liquid covering at least one part        of the membrane so as to produce an aperture having a first        diameter (zero or not), and    -   an actuation situation when a non-zero electrical voltage is        applied to the actuation device, the central part of the        membrane having a curvature different to the curvature at rest,        the opaque liquid covering at least one part of the membrane so        as to produce an aperture having a second diameter (zero or not)        different to the first diameter (greater or less than the        latter).

The opaque liquid is for example liquid or oil comprising pigmentsand/or dyes in sufficient amount to block the incident light beam. Forexample, the opaque liquid can be selected from the following liquids:propylene carbonate, water, an index liquid, optical oil or ionicliquid, a silicone oil, an inert liquid with considerable thermalstability and low saturating vapor pressure.

The wettability of the opaque liquid relative to the membrane can beadjusted by selecting a material adapted for the membrane and/or byapplying hydrophobic or hydrophilic surface treatments to the membrane.Such treatments are known per se and therefore will not be described indetail here. Reference could be made for example to the followingdocuments: [11] for the effect of plasma treatments; [12] for the effectof the surface state and roughness of materials; [13] for examples ofmaterials (such as particularly hydrophobic Cytop™ and hydrophilicSiO₂).

In an embodiment illustrated in FIGS. 5A and 5B, the opaque liquid isnot encapsulated in a dedicated cavity and is simply in contact withambient air, which constitutes the second transparent fluid mentionedearlier.

This embodiment can be obtained simply by depositing the opaque liquidonto the outer face of the membrane of an optical device with variablefocal length.

The spreading of the opaque liquid 3 on the membrane 1 can be adjustedand controlled as a function of the deposited volume of liquid, itswettability on the membrane, surface preparation of the membrane, itsstructuring or again the initial deformation of the membrane.

As mentioned earlier, the configuration of the opaque liquid at rest isnot necessarily a uniform layer of thickness. On the other hand, thedeformable membrane is not necessarily planar in the rest situation.Finally, the position of the actuation device illustrated in thedifferent figures is not limiting. So, the actuators can be deflectedupwards or downwards at rest independently of the curvature of thecentral part of the membrane.

FIGS. 6A-6D illustrate non-limiting different configurations of thedevice 100 at rest, that is, in the absence of application of electricalactuation voltage. It should be noted that these configurations can alsobe found in an embodiment in which constant volumes of the opaque liquidand of the second transparent fluid are encapsulated in a cavity.

In the case of FIG. 6A, the wettability of the opaque liquid relative tothe membrane—which is planar in this embodiment—and of the peripheralsupport 12 is such that the opaque liquid 3 does not form a layer ofuniform thickness but a layer of concave form whereof the thickness isgreater at the center than at the periphery of the membrane. As thelayer of opaque liquid 3 is continuous, the aperture of the opticaldevice 100 is zero.

In the case of FIG. 6B, the wettability of the opaque liquid relative tothe membrane—which is planar in this embodiment—and the peripheralsupport 12 is such that the opaque liquid 3 does not form a layer ofuniform thickness but a layer of convex form whereof the thickness isgreater at the periphery of the membrane than at the center. As thelayer of opaque liquid 3 is continuous, the aperture of the opticaldevice 100 is zero.

In the case of FIG. 6C, the membrane is not planar but its central part1 a has a concavity for receiving the opaque liquid 3. The surface ofthe opaque liquid 3 opposite the membrane is planar as such. As thelayer of opaque liquid 3 is continuous, the aperture of the opticaldevice 100 is zero.

In the case of FIG. 6D, the membrane is convex in its central part 1 a,such that the opaque liquid 3 extends on either side of the apex of thecentral part 1 a. The aperture of the optical device 100 is thereforenon-zero.

However, this embodiment can be sensitive to gravity. In effect, as afunction of the volume of opaque liquid, the geometry of the membraneand the wettability between the opaque liquid and the deformablemembrane, such an embodiment can create a device having differentoptical performance depending on its orientation.

To avoid this problem and limit the effects of gravity onelectro-optical performance of the device, transparent fluid of the samedensity as the opaque liquid can advantageously be utilized.

In addition, the embodiment of FIGS. 5A and 5B is likely to also haveanother disadvantage. In effect, the potential difference in refractionindex between the first transparent fluid and the ambient air generatesa variation in focal length of the device coupled with the variation inoptical aperture.

To prevent such an effect, the first and second transparent fluids canbe selected to have identical refraction indices (for example using thesame gas or same liquid). This also avoids deviating from the variationin focal length due to deformation of the membrane between the reststate and actuation state (cf. FIGS. 1A and 1B for example), if thevariation in focal length is not intended and the aim is only to varythe aperture.

To simultaneously optimize transmission of the assembly, a transparentfluid or fluids having a similar refraction index or even near themembrane and the substrate or substrates can be selected.

For these reasons, a preferred embodiment of the invention relates to anoptical device in which the second transparent fluid is encapsulated atconstant volume in a second cavity with the opaque liquid. So, thesecond transparent fluid (advantageously identical to first transparentfluid) can be selected and the behavior of opaque liquid under theeffect of gravity can be best controlled.

To form such a cavity, a second substrate such as shown in FIGS. 1A to4C is advantageously used.

At rest (zero actuation voltage), the rest position of the membrane andthe respective volumes of opaque liquid and second fluid and theirposition in the optical device determine an initial aperture of thedevice.

To make variation in aperture easier and adapt the accessible range ofaperture, different configurations are possible in addition to theconfigurations illustrated in FIGS. 6A-6D described above.

FIGS. 7A and 7B illustrate two embodiments of an optical device 100 atrest (actuation voltage zero). In both cases, the membrane 1 is planarat rest. In the case of FIG. 7A, the second transparent fluid 4 hasgreater wettability relative to of the second substrate 13 than in thecase of FIG. 7B. This induces a smaller contact angle between the volumeof second transparent fluid and the surface of the second substrate. Theresult of this, in the case of FIG. 7A, is a zero aperture of theoptical device 100, while the aperture has a non-zero diameter d0 in thecase of FIG. 7B.

The second substrate 13 can be functionalized to determine the spread ofthe second transparent fluid on contact. In terms of functionalizing,any localized surface treatment intended to adjust the wettability ofthe second fluid on the second substrate can especially be cited. Thelocal contribution of hydrophilic/hydrophobic material on the secondsubstrate can also be envisaged.

A third embodiment forms a network of diaphragms with variable aperture.

For this purpose, as illustrated in FIGS. 8A-8B, the membrane 1comprises a stiffening structure 14 which delimits, in the central partof the membrane, at least two elementary deformable regions and whichdefines the mechanical behavior of the membrane (especially itsstiffness) in the regions of the central part of the membrane extendingbetween said elementary deformable regions.

According to an embodiment, the stiffening structure can comprise aplurality of grooves which extend perpendicularly to the surface of themembrane.

Alternatively, the stiffening structure can comprise a layer extendingover the central part of the membrane and having apertures delimiting atleast two deformable regions of the membrane.

The use of a stiffening structure in the form of grooves is particularlypreferred for making a large number of elementary deformable regions inthe central part of the membrane. The small thickness of the grooves infact maximizes the number of separate deformable regions in the centralpart of the membrane.

Conversely, the use of a stiffening structure in the form of a layerhaving openings is preferred for making a small number of elementarydeformable regions.

Advantageously, the stiffening structure is arranged so as to formcells, the portion of membrane located inside each cell beingdeformable.

Each portion of membrane located inside a cell is capable of deformingreversibly, from a rest position (which can be planar or not), under theaction of displacement of the first transparent fluid, which varies thefluid thickness at the level of the central part of each membrane. Saidportions of membrane can have identical stiffness from one region of themembrane to the other or by contrast have different stiffness, saidstiffness being especially able to be adjusted by a local change inthickness or material of the membrane.

The opaque liquid is in contact with the deformable membrane, forexample to the side of the stiffening structure 14.

The second transparent fluid 4 (which in this case is a liquid) isplaced on the second substrate 13 prior to assembly of the device 100 inthe form of a network of droplets facing the network of cells of thedeformable membrane. This arrangement in droplets is achieved by locallyadjusting the wettability of the second transparent fluid relative tothe second substrate.

A network of elementary optical devices with variable aperture is formedin each cell.

At rest (FIG. 8A), the opaque liquid 3 covers the entire surface of eachcell. Each elementary optical device has a zero aperture.

With application of electrical actuation voltage (FIG. 8B), eachelementary deformable region of the membrane deforms followingdisplacement of the first transparent fluid 2 by the actuation device 5.The result is that the opaque liquid is pushed towards the periphery ofeach cell, accordingly placing the membrane in contact with the secondtransparent fluid and producing a non-zero aperture of diameter D0.

In the example illustrated in FIG. 8B, the aperture diameter d0 isidentical for all elementary diaphragms of the network during actuation.However, adjusting the volume and/or the shape of the network ofdroplets of the second transparent fluid can produce different aperturediameters on the network of diaphragms during actuation.

According to another embodiment illustrated in FIGS. 8A-8B, the networkof diaphragms can be obtained without having the second transparentfluid in the form of a network. Integrating the second fluid in oneamount can also create a network of diaphragms.

As evident in FIG. 9A, which shows the optical device 100 at rest, thesecond transparent fluid 4 is in the form of a continuous layer coveringthe surface of the second substrate 13. The opaque liquid 3 covers thesurface of the membrane in all the cells defined by the stiffeningstructure 14.

With application of electrical actuation voltage (FIG. 9B), eachelementary deformable region of the membrane deforms followingdisplacement of the first transparent fluid 2 by the actuation device 5.The result is that the opaque liquid is pushed towards the periphery ofeach cell, accordingly placing the membrane in contact with the secondtransparent fluid and producing a non-zero aperture at least in somecells.

In the example illustrated in FIG. 9B, the aperture diameter of thenetwork of diaphragms is not identical from one cell to the other. So,an aperture having a diameter D2 is obtained in the cells locatedclosest to the center of the membrane, a diameter D1 less than D2 isobtained in the cells enclosing said central cells, and no aperture iscreated in the cells located at the periphery of the network. However,adjusting the volume or form of the spread of the second transparentfluid can create the same aperture diameter on the network of diaphragmsduring actuation.

Another solution to create a network of diaphragms having differentdiameters in an actuation situation consists of using an optical device100 devoid of the stiffening structure 14 and arranging the secondtransparent fluid 4 according to a network of droplets.

FIGS. 10A and 10B illustrate such a device 100 respectively at rest andin the actuated state.

At rest, the membrane 1 is planar and the opaque liquid covers theentire surface of the membrane 1. The optical device therefore has azero aperture.

Under the effect of electrical actuation voltage, the central part 1 aof the membrane deforms and makes contact with at least the droplets ofsecond transparent fluid 4 located at the center of the second substrate13. In this case, the elementary optical devices have an aperture ofnon-zero diameter, the diameter being all the greater since theelementary optical device is near the center of the device 100. However,for elementary optical devices located at the periphery, the membrane 1remains in contact with the opaque liquid 3, such that said devices havea zero aperture.

The optical device such as described hereinabove can be made by means ofmicromanufacturing techniques.

In particular, the manufacturing method can comprise the followingsteps.

In reference to FIG. 11A, a device is provided with variable focallength comprising the deformable membrane 1 anchored between theperipheral supports 10, 12, the actuation device 5 and the firsttransparent liquid 2 encapsulated between the membrane 1, the peripheralsupport 10 and the first substrate 11. The manufacturing of such adevice is known per se, especially from documents [5]-[10].

Next, in reference to FIG. 11B, a determined volume of the opaque liquid3 is dispensed on the deformable membrane 1. Previous surface treatmentcan be carried out if necessary to optimize the wettability of theopaque liquid on the deformable membrane.

In reference to FIG. 11C, the second substrate 13 is provided on which aperipheral adhesive bead 14 is deposited, for example by serigraphy, andthe second transparent fluid 4 (here liquid) is dispensed on the secondsubstrate in the form of a single drop (case of FIG. 11C) or a networkof drops (not illustrated) according to the embodiment in question. Saidsubstrate can have undergone surface treatment adapted to adjust thewettability of the second transparent fluid.

Next, in reference to FIG. 11D, the second substrate 13 is stuck on tothe device with variable focal length by means of the bead 14 of FIG.11B so as to encapsulate the opaque liquid 3 and the second transparentfluid 4. The encapsulation method used is well known in the prior art,especially methods used to encapsulate liquid crystals in LCD screens.The method described in FIG. 11D is known under the name of “One DropFilling” (adhesion on liquid). In the event where the second transparentfluid is gas, a classic adhesion method is used. Said adhesion can benon-hermetic if said fluid is air (air can freely enter or leave thesecond cavity). In the event where the second transparent fluid isdifferent to ambient air, said adhesion must however be hermetic.

As the second transparent fluid 4 and the opaque liquid 3 arenon-miscible, they form two separate entities in the resulting cavity.

The optical device 100 illustrated in FIG. 11E is the result.

The invention therefore provides an optical device with compact variableaperture, of low power consumption and easy to manufacture by means ofcollective microsystems methods.

In this respect, such a device is particularly adapted to miniaturecameras for mobile telephony.

Other advantageous applications relate to the industry, the medicalfield, automobile field, security and defense.

The present invention can also apply in the field of lighting or evendisplay.

REFERENCES

-   [1] U.S. Pat. No. 21,470-   [2] “Sliding-blade MEMS iris and variable optical attenuator”,    Journal of Micromechanics and Microengineering, 14:1700-1710, 2004-   [3] US 2015/037024-   [4] Thesis by Philipp Müller, “Tunable optofluidic apertures”,    Research in Micro-optics, Volume 11, edited by Prof. Dr. Hans Zappe,    Department of Microsystems Engineering—IMTEK, University of    Freiburg, 2012, paragraph 1.2-   [5] FR 2919073-   [6] FR 2919074-   [7] FR 2930352-   [8] FR 2938349-   [9] FR 2950153-   [10] FR 2950154-   [11] “Wettability Tests of Polymer Films and Fabrics and    Determination of Their Surface Energy by Contact-Angle Methods”,    Daphne Pappas, Craig Copeland, Robert Jensen, ARL-TR-4052, March    2007-   [12] “Wettability Switching Techniques on Superhydrophobic    Surfaces”, Nanoscale Res Lett (2007) 2:577-596-   [13] “Electrowetting: from basics to applications”, J. Phys.:    Condens. Matter 17 (2005) R705-R774

The invention claimed is:
 1. An optical device with variable aperture,comprising: a deformable membrane comprising a central optical area; asupport to which a peripheral anchoring area of said membrane isconnected; a first cavity filled with a constant volume of a firsttransparent fluid in a determined range of wavelengths, said cavitybeing delimited at least in part by a first face of said membrane and awall of the support; and at least one actuation device of a region ofthe membrane located between the peripheral anchoring area and thecentral optical area of the membrane, configured to bend said region ofthe membrane by application of electrical actuation voltage to displacesome of the volume of the first fluid towards the center or towards theperiphery of the first cavity, said displacement of fluid to deform thecentral area of the membrane, wherein a constant volume of an opaqueliquid in said determined range of wavelengths is in contact at leastlocally with a second face of the membrane opposite the first face andwith a second fluid transparent in said determined range of wavelengthsand non-miscible with said opaque liquid.
 2. The device of claim 1,wherein the volume of opaque liquid is selected so that: in a restsituation when no electrical voltage is applied to the actuation device,the opaque liquid covers at least one part of the membrane to produce anaperture having a first diameter, and in an actuation situation whennon-zero electrical voltage is applied to the actuation device, thecentral part of the membrane having a curvature different to thecurvature at rest, the opaque liquid covers at least one part of themembrane to produce an aperture having a second diameter different fromthe first diameter.
 3. The device of claim 1, further comprising asecond cavity opposite the first cavity relative to the membrane, saidsecond cavity containing the opaque liquid and a constant volume of thesecond transparent fluid.
 4. The device of claim 3, wherein the opaqueliquid and the second transparent fluid have substantially the samedensity.
 5. The device of claim 3, wherein the first and secondtransparent fluids have substantially the same refraction index.
 6. Thedevice of claim 3, wherein the first and the second cavity have atransparent wall opposite the membrane.
 7. The device of claim 6,wherein the transparent wall of the first cavity and/or of the secondcavity comprises an optical filter on its face opposite the cavity. 8.The device of claim 6, wherein the transparent wall of the first cavityand/or of the second cavity comprises a fixed optic on its face oppositethe respective cavity.
 9. The device of claim 6, wherein the transparentwall of the first cavity and/or of the second cavity comprises a devicewith variable focal length.
 10. The device of claim 9, wherein said wallhas a central aperture and said device with variable focal lengthcomprises: a deformable membrane closing said aperture, a peripheralarea of the deformable membrane being anchored on said wall; and atleast one actuation device of a region of the membrane located betweenthe peripheral anchoring area and the central area of the membrane,configured to bend said region of the membrane by application ofelectrical actuation voltage to displace some of the volume of the fluidtowards the center or towards the periphery of the cavity.
 11. Thedevice of claim 3, wherein the membrane comprises a stiffening structurecomprising cells which delimit, in the central optical area of saidmembrane, at least two deformable regions.
 12. The device of claim 11,wherein the second transparent fluid is arranged in the second cavity inthe form of a plurality of elementary volumes each arranged facing arespective cell.
 13. The device of claim 11, wherein the secondtransparent fluid is arranged in the form of a single continuous volumefacing the cells.
 14. The device of claim 3, wherein the secondtransparent fluid is arranged on the wall of the second cavity oppositethe deformable membrane in the form of a plurality of elementaryvolumes.
 15. The device of claim 1, wherein the actuation device ispiezoelectric.
 16. A method of manufacturing an optical device withvariable aperture, comprising: providing an optical device with variablefocal length comprising a deformable membrane, an actuation device, anda first transparent fluid in a first cavity; dispensing a determinedvolume of an opaque liquid on the deformable membrane; providing asecond substrate and dispensing a second transparent fluid on saidsecond substrate in the form of at least one drop; and adhering thesecond substrate on the optical device with variable focal length toencapsulate the second transparent fluid and the opaque liquid betweenthe second substrate and the deformable membrane.
 17. The method ofclaim 16, further comprising: adjusting wettability of the opaque liquidby applying a surface treatment to the deformable membrane, wherein thesurface treatment comprises a hydrophobic surface treatment or ahydrophilic surface treatment.
 18. The method of claim 16, furthercomprising: adjusting wettability of the second transparent fluid byapplying a surface treatment to the second substrate, wherein thesurface treatment comprises a hydrophobic surface treatment or ahydrophilic surface treatment.
 19. The method of claim 16, furthercomprising: delimiting, with a stiffening structure, at least twodeformable regions in a central optical area of the membrane.
 20. Themethod of claim 16, wherein the first transparent fluid and the secondtransparent fluid have a similar refraction index.